The present invention relates to an information recording and reproducing technology for optical discs, and relates to a header region evaluation circuit, an optical disc apparatus, and a header region evaluation method that evaluate a physical header region formed of pre-pits.
In DVD-RAM (Digital Versatile Disk Random Access Memory), both lands (inter-grooves) and grooves are used as a recording and reproducing region (information recording unit). Recording tracks formed in a spiral are divided into land tracks having the lands as recording regions and groove tracks having the grooves as the recording regions. The land and the groove tracks are switched every revolution of an optical disc. Each of the land and the groove tracks has an integer number of recording sectors per revolution of the optical disc and a physical header region representing address information and the like. The physical header region is provided before each recording sector and includes four each of the physical address regions. Two of the four physical address regions are provided on inner half of the track pitch from the center of the track, while the remaining two physical address regions are provided as embossed pits at the position offset to an outer circumference side from the center of the track by half the track pitch.
Offset polarities of the physical header regions can be used to distinguish the land and groove tracks. In the physical header region preceding the recording sector on the land track, first two of the four physical address regions are offset to the inner circumference side, and the last two physical address regions are offset to the outer circumference side. On the contrary, in the physical header region preceding the recording sector on the groove track, first two physical address regions are offset to the outer circumference side, and the last two physical address regions are offset to the inner circumference side. The optical disc apparatus uses a difference in amounts of reflected light received by a light receiving element including light receiving units divided into two along the tracks as a tracking error signal and a wobble signal. An offset in the physical header region can be detected by this difference in the amounts of reflected light. Thus, the difference in the amounts of reflected light is used to evaluate the land and groove tracks.
For example, Japanese Unexamined Patent Application Publication No. 2000-200438 discloses an optical disc tracking method and an optical disc apparatus using the same that offset the physical header region from the center of the tracks and use a disposed identification signal in order to determine whether it is the land or the groove track, and then switch tracking polarities according to the determination result. An operation of an optical disc apparatus 90 is explained with reference to FIG. 13.
A light receiving element including light receiving units that are divided into two in the direction along tracks of the optical disc 1 are mounted on a pickup 2. An output from the light receiving element is input to a sum signal detection unit 3 and a difference signal detection unit 4 and is respectively output as a signal proportional to a sum of the amounts of reflected light and a signal proportional to a difference in the amounts of reflected light. A sum signal output from the sum signal detection unit 3 is converted into binarized data by a reproducing signal processing unit 5 and is used to reproduce data recorded on the optical disc 1 or reproduce address information and the like from the embossed pits.
An outline of the process for a difference signal output from the difference signal detection unit 4 is explained using FIG. 14. The difference signal is input to a waveform shaping unit 7 and binarized by two comparators 701 and 702 including thresholds Lth and Rth. A shaping signal L0 obtained by binarization with the threshold Lth and a shaping signal R0 obtained by binarization with the threshold Rth are converted into delay signals L1 and R1. The delay signals L1 and R1 are switched to the level “1” in the difference signal processing unit 91 when a pulse train continues for a certain period (t1) and then after an elapse of a certain period (t2), switched to the level “0”. Further, in the difference signal processing unit 91, the delay signal L1 is latched at a rising edge of the delay signal R1 from the level “0” to the level “1” and is output as a groove detection signal. Conversely, the delay signal R1 is latched at a rising edge of the delay signal L1 and output as a land detection signal. When either one of the groove detection signal and the land detection signal rises from the level “0” to the level “1”, a header detection gate is reset to the level “0”, and after an elapse of a period (t3) corresponding to the recording region of the recording sector, the header detection gate is switched to the level “1” again. Both the groove detection signal and the land detection signal are reset to the level “0” when a header detection signal is switched to the level “1”.
Japanese Unexamined Patent Application Publication No. 2000-200438 indicates that it is possible to prevent false detection in the recording and reproducing region by detecting the pulse train in the shaping signals L0 and R0 only in the period when the header detection gate is the level “1” and excluding pulses appearing in the period when the header detection gate is the level “0” as noise. The information reproduction unit 92 performs a demodulation process and an error correction process based on the binarized data output from the reproducing signal processing unit 5 and the header detection gate output from the difference signal processing unit 91 and outputs reproduced data. The address reproduction unit 93 extracts and demodulates the address information on the embossed bits recorded in the physical address region based on the binarized data and the groove detection signal, the land detection signal, and the header detection gate output from the difference signal processing unit 91 and outputs the address information to the system control unit 11.
In response to the groove detection signal, the land detection signal, the header detection gate, and a signal indicating tracking on/off passed from the system control unit, a polarity control unit 94 selects the tracking polarity. The polarity selection unit 13 reverses the polarity of the difference signal output from the difference signal detection unit 4 according to the tracking polarity selection result by the polarity control unit 94 and outputs the difference signal to the tracking control unit 14. The tracking control unit 14 switches the tracking polarities according to the polarity selected by the polarity selection unit 13 and controls the pickup 2.
Such a configuration enables determination of whether a following track is the land or groove track by the offset in the physical header region and also enables correct tracking of the land and groove tracking by switching the tracking polarities and controlling the position of the pickup.
Moreover, in regard to tracking control of an optical disc apparatus, Japanese Unexamined Patent Application Publication No. 2002-288855, for example, discloses an apparatus capable of suppressing unstable tracking control due to an influence of an adjacent track and off-tracking.
As mentioned above, on DVD-RAM, the physical header regions formed by the embossed pits are offset to the inner and outer circumference sides relative to the center of the tracks. This offset polarity enables the evaluation of whether the following recording region is on the land track or groove track.
Guiding grooves of the land and groove tracks are wobbled sinusoidally on the recording regions of DVD-RAM. Further, disturbance by a contrast of a recording mark is also included in the difference signal in the recording region where data is recorded. Furthermore, a tracking error signal component indicating displacement of a focused beam emitted from the pickup 2 on the optical disc from the center of the tracks.
Therefore, a large offset often appears in the difference signal when a shock is applied to the optical disc 1.
In addition, when a tilt of the pickup 2 is not optimized, a signal amplitude obtained from a physical address offset to the inner circumference side and a signal amplitude obtained from a physical address offset to the outer circumference side may not be balanced.
In the physical header region formed by the embossed pits, usually a large difference signal amplitude than the recording mark is obtained.
However, disturbance and an imbalanced amplitude in the physical address makes it difficult to ensure an enough detection margin in the threshold.
There is a problem that when noise is generated in the difference signal due to a defect and the like while the detection margin is small, the physical header region is incorrectly evaluated.
Thus, there is disturbance by the contrast of the recording mark, a low-frequency offset caused by unstable tracking, and fluctuation caused by a defect in the difference signal that is used to detect the offset polarity, thereby hindering the evaluation of the physical header region.
In particular, incorrect evaluation of the land and groove tracks reverses a tracking servo polarity, and this leads to incorrect tracing of the recording and reproducing tracks.
Japanese Unexamined Patent Application Publication No. 2000-200438 describes a function that limits the period to detect the pulse train in the shaping signals L0 and R0 by the header detection gate and removes the pulse train appearing in the recording region of the recording sector as noise. Specifically, the detection gate is closed for the period corresponding to the recording region of the recording sector based on the header detection gate detected previously while tracing the recording track. This enables exclusion of the pulse train appearing due to a defect in the recording region. However, such a header detection gate is not effective in the case of detecting the physical header region immediately after a seek operation. Specifically, the header detection gate cannot be created based on the physical header region detected last time after the pickup 2 moves between the land and groove tracks during the seek operation. Moreover, as the pickup 2 crosses the plurality of tracks during the seek operation, large noise is generated in the difference signal. Thus, there is large noise in the difference signal immediately after the seek operation and the protection by the header detection gate cannot be expected, thereby generating a problem that makes it difficult to accurately detect the physical header region.
Additionally, Japanese Unexamined Patent Application Publication No. 2002-288855 discloses a method for separating land pre-pits adjacent on the inner and outer circumference sides of the tracks using a high pass filter and detecting an amplitude in order to suppress unstable tracking control and off-tracking. However, this technique does not aim to reduce the false detection of the physical header region.